Microelectronic devices, such as various types of integrated circuits, are generally hermetically sealed in a package to protect the microelectronic device from being damaged and from the ambient. One type of package used comprises a ceramic housing which has a recess in a surface thereof in which the microelectronic device is mounted, and a cover plate extending over the recess and hermetically sealed to the housing. The cover plate may be a metal plate or a plate of an insulating material, such as glass or ceramic. The cover plate is generally sealed to the housing by a solder which adheres to the material of both the housing and the cover plate and which will withstand the operating temperature of the microelectronic device. Often, the housing and/or cover plate is provided with an adherent metal layer to which the solder will bond so as to provide the desired hermetic and mechanical bond between the housing and cover plate.
A problem with using solder for sealing the cover plate to the housing arises from the high temperature required to melt the solder when bonding the cover plate to the housing. There are some microelectronic devices that cannot stand such high temperature and which can be damaged during the bonding of the cover plate to the housing. For example, certain types of solid state image sensors include an optical filter layer or lenslets of a resin which will melt or degrade at the high temperatures required for melting solders. Therefore, it is desirable to have a sealing means for bonding a cover plate to a housing of a microelectronic package which can achieve the bond at a low temperature which does not adversely affect the microelectronic device in the package. One such sealing means is described in U.S. Pat. No. 4,895,291 to E. J. Ozimek et al, issued Jan. 23, 1990 and entitled "Method of Making a Hermetic Seal in a Solid-State Device". However, a problem with the sealing means described in this patent is that it melts at the same low temperature at which it is formed. Thus, if the microelectronic device is subjected to that low temperature the bond is destroyed. Therefore, the sealing means not only must be capable of being formed at relatively low temperatures and provide both a good mechanical and hermetic seal between the cover plate and housing, but also must be capable of withstanding the operating temperature of the microelectronic device which can be higher than the temperature at which the bond is initially formed.
In our copending U.S. patent application Ser. No. 594,789), which is being filed concurrently with the present application and which is entitled "Hermetically Sealed Microelectronic Package and Method of Making the Same", there is described a seal for a microelectronic package which can be formed at relative low temperatures, but which withstands much higher temperatures. The seal of this copending patent application comprises the use of an amalgam to bond the cover plate to the housing of the package and form a hermetic seal therebetween. The amalgam comprises a mixture of a liquid metal, such as mercury or gallium, and a powdered metal or combination of powdered metals, such as powders of copper, nickel, cobalt, gold, silver, tin, magnesium, antimony, manganese, chromium, titanium, molybdenum, aluminum, iron and combinations of these metals. A problem with the use of an amalgam is that the consistency of the amalgam makes it diffcult to apply it to the parts to be bonded together and particularly to apply an uniform amount of the amalgam to the parts. Therefore, it is desirable to have an amalgam in a form which can be easily applied to the parts to be bonded together, such as the cover plate or housing of a microelectronic package, in a layer of uniform thickness to achieve a good bond between the parts.
As shown in U.S. Pat. No. 4,328,921 (S.C. Liang, issued May 11, 1982), entitled "Attachment of Solder Preform to a Cover For a Sealed Container", solders have been formed into preforms of the desired shape and thickness to be used in the bonding of two parts together. However, an amalgam is in a liquid state near room temperature so that it cannot be formed into a preform in the same manner as a solder. Therefore, it would be desirable to have a method of forming an amalgam into a preform to allow it to be used more easily to bond two parts together.